1. Field of the Invention
The present invention relates to a purification of hexafluoropropylene oxide. More particularly, it relates to a separation of hexafluoropropylene oxide from hexafluoropropylene by an extractive distillation.
2. Description of the Prior Art
Hexafluoropropylene oxide (hereinafter referring to as HF PO) can be obtained by an epoxidation of hexafluoropropylene (hereinafter referring to as HFP) in various methods such as the autoxidation and the organic hydroperoxide method.
In these methods, HFPO can be obtained in high selectivity when the reaction condition is adjusted to give a low conversion of HFP. However, the selectivity to HFPO is lowered depending upon an increase of the conversion of HFP. In order to effectively utilize the starting material of HFP, it is preferable that the reaction is stopped under the condition maintaining high selectivity of HFPO and the product is separated from the starting material and the unreacted HFP is recycled to the reaction system.
The crude product obtained by the oxidation contains hexafluoropropylene (HFP) together with several by-products. Some of the by-product may be removed without difficulty by the conventional techniques of scrubbing and distillation, however, the boiling point of HFP is too close to that of HFPO (-29.4.degree. C. and -27.4.degree. C. respectively) to permit the separation by the conventional distillation.
As a method of separating HFP from HFPO, it has been known that HFP is brominated to form the corresponding dibromo compound which has a boiling point being remarkably different from that of HFPO. The dibromo compound is separated by a distillation and then, the dibromo compound is treated with zinc powder and HFP is recovered.
However, the method has disadvantages of a loss of HFP and a necessity for costly processing to remove the reacted material.
In order to overcome the disadvantages, the extractive distillation using a solvent for lowering volatility of HFP to increase the relative volatility of HFPO has been proposed (Japanese Patent Publication No. 14933/1967 and U.S. Pat. No. 3,326,780).
The following solvents has been proposed as suitable solvents in the extractive distillation mono-, di- and tri-substituted benzenes wherein the substituent is an alkyl groups of 1 to 4 carbon atoms, an alkoxyl group of 1 to 4 carbon atoms; dialkyl ethers of ethylene glycol or diethylene glycol wherein the alkyl group of the ether has 1 to 2 carbon atoms; dialkyl carbonates wherein the alkyl group has 1 to 4 carbon atoms; carbon tetrachloride; and chloroform.